Waldenstrom's macroglobulinemia (WM) is a distinct clinicopathological entity resulting from the accumulation, predominantly in the bone marrow, of clonally related lymphoplasmacytic cells which secrete a monoclonal IgM protein. This condition is considered to correspond to lymphoplasmacytic lymphoma (LPL) as defined by the World Health Organization classification system. Genetic factors play an important role in the pathogenesis of WM, with 25% of patients demonstrating a family history. IgM monoclonal gammopathy of unknown significance (MGUS) often precedes the development of WM. The primary oncogenetic event resulting in malignant transformation in WM remains to be delineated. Knowledge of such genomic alteration(s) may permit advances in diagnostic testing, and development of targeted therapies.